The present invention relates to methods for manufacturing microcrystalline cubic boron-nitride-layers through chemical vapor deposition in an electromagnetic alternating field (PECVD-method).
For a number of technical applications, boron-nitride layers are of great interest. For example, boron-nitride layers have applications as insulating layers in integrated circuits. For intermetal insulating and final passivation layers for use in VLSI semiconductor circuits, dielectric layers having small dielectric constants .epsilon..sub.r, good insulating and blocking properties, and high breakthrough stability, are necessary. Additionally, it is necessary that the layers have an optimally conformal edge covering and are not hygroscopic. Boron-nitride layers are suitable for these requirements, particularly because they have a sufficiently low dielectric constant in either an amorphous or polycrystalline form.
The article of W. Schmolla and H. Hartnagel, in Solid State Electronics 26, No. 10, page 931 to 939, discloses that for the manufacture of crystalline boron-nitride layers, relatively high temperatures are necessary. Additionally, the article discloses that because of the increased energy gap, improved insulating properties are to be expected with cubic crystalline boron nitride as compared with other crystalline forms.
Known methods for the production of cubic boron nitride are based on a multistage process. First, boron nitride in a different crystalline form, principally in a hexagonal or rhombohedral phase, is generated. This is then converted into a cubic phase using a high temperature and high pressure (see, e.g., EPA 0 216 932).
Even for the production of the rhombohedral or hexagonal phase, typically, temperatures above 750.degree. C. are necessary. Because of such temperatures, these processes are not viable for the production of intermetal insulating and final passivation layers for VLSI semiconductor circuits.
The article of A. Chayahara et al. in Appl. Surface Science 33/34 (1988) p. 561-566, suggests a method for manufacturing cubic boron nitride using B.sub.2 H.sub.6 and nitrogen in a PECVD-method. This method is not suitable for manufacturing applications since it has a deposition rate of less than 6 nm/min. To this end, typical layer thicknesses of a few 100 nm are required for use as an insulation layer. Another disadvantage of the method is that it uses a very dangerous gaseous starting substance B.sub.2 H.sub.6.
Another application of boron-nitride layers is for mask membranes in x-ray lithography. For manufacturing of so-called VLSI-components with structure sizes below 0.5 .mu.m, the application of x-ray lithography is necessary. An x-ray mask requires the use of a thin membrane, that is highly permeable to soft x-rays. Additionally, the membrane must be dimensionally stable during the manufacturing process and during its use for the component manufacturing. As revealed in the article W. Johnson et al. in Journal of Vacuum Science and Technology B5, January 1987, p. 257 to 261, amorphous boron nitride is basically well suited for use as a mask membrane. It can be produced, for example, using the method described in the article of Schmolla and Hartnagel set forth above.
There are some disadvantages, however, in the use of boron nitride as a mask membrane. In air, the resultant boron nitride layers change their surface and structure--due, for example, to boric acid crystal growth. The reduced transmission of the layers connected therewith makes the optical adjustment of the mask difficult when it is used for component manufacturing. Local changes of mechanical tensions in the boron nitride layer can cause a displacement of the mask. This results in adjustment inaccuracies that are not tolerable.
Another disadvantage is the complicated system structure that is required. Further, the starting substances used for the manufacturing method are dangerous.
A still further, application of boron-nitride layers is for coating hard material. Cubic boron nitride is principally used for the production of grinding discs and turning tools, as well as for the processing of hardened steel, tool steel, super alloys, and chrome nickel steel.
The hardness of cubic boron nitride layers is preserved up to approximately 600.degree. C. In contrast, the hardness of, for example, tungsten carbide considerably decreases at approximately 300.degree. to 400.degree. C. (see, e.g., M. Rand, J. Roberts in Journal Electrochem. Soc. 115, 1968, page 423). Such layers are principally generated from the hexagonal phase of the boron nitride at temperatures of above 1500.degree. C. and a pressure of above 80 kbar. The disadvantages of this method include the necessity of at least two unit processes and the high temperatures that are necessary, which change the structure of the workpiece to be hardened.